Recently, display devices comprising a thin-film transistor as a semiconductor device have been put into practical use. As examples of display devices, a liquid crystal display device and an organic electroluminescence display device are considered.
These types of display devices comprise an array substrate comprising, for example, a plurality of thin-film transistors (TFTs), signal lines, gate lines, an interconnect portion of source and drain electrodes, and a passivation film. The thin-film transistors and various interconnect portions are patterned in a predetermined shape by photolithography, dry etching, etc.
For example, in general, in a bottom-gate channel-etch TFT, a semiconductor layer is processed in an island shape. Subsequently, an interconnect layer is formed on the semiconductor layer. Further, the interconnect layer is etched to form signal lines and scanning lines. Subsequently, a passivation film is formed in order to cover the signal lines, the scanning lines and the semiconductor layer.
However, when a Ti-based/Al-based/Ti-based (TAT) laminated film is used as the interconnect layer and the signal line in order to obtain a high-definition display device, out of the sidewalls of the signal line formed by etching, the sidewall located in an end portion of the semiconductor layer and the sidewall located apart from the semiconductor layer easily have a large taper angle. When the taper angle of sidewalls is large, the taper angle of the portion which covers the sidewalls of the signal line is also large in the passivation film formed on the interconnect layer. Thus, a cavity (fine hole) is easily formed in the standing portion. In this case, the semiconductor layer of the TFT is reduced by water or hydrogen inserted from outside through the cavity (hole). As a result, the TFT easily becomes conductive.